Very unreliable means have hitherto been used for detecting the correct coordination of two separate data carriers, more particularly cinema films and the associated recording tape. In the simplest case, identifying markings were inscribed on the reel or cassette supporting each data carrier. Often the inscription on the sound carrier or the image carrier could not be read in the darkened room in which the performance was given so that mutually unrelated tapes and film were played back. Moreover, the inscription on the reel or on the cassette became very unclear and not readily readable in the course of time or became completely detached from the data carrier.
An improvement was obtained in recent times with a synchronizing method between film and sound, known as the "PIP" method, according to which the coordination between the two strips is indicated thereon by means of marks. To this end the marks on the sound carrier control the operating frequency of a transport claw for the drive of the image carrier whose driving speed may vary from zero or very low values in order to project a stationary image or to show a scene with "slow motion" effect, to normal projection speed. If a recording tape that was not correlated with the film was erroneously played back, the absence of correlation was noted sooner or later only by reference to the contrast between the sound and the film. In some circumstances it was possible that the wrong coordination went completely undetected, particularly in films with a minimum scene content and musical sound background. However, this could have a very detrimental effect especially if the sound background intended for the film was to have been particularly informative.